SCIENTIFIC CULTURE, 591 



student a knowledge of the three great laws of Mariotte, of Charles, 

 and of Avogadro. He must be made to understand how molecules are 

 defined by the physicist, and how their relative weights may be in- 

 ferred by a comparison of vapor densities. He should then be made 

 to compare the relative molecular weights, deduced by physical means, 

 with the definite proportions he has observed in chemical processes. 

 He will thus himself be led to the conclusion that these definite pro- 

 portions are the proportions of the molecular weights, and that the 

 constancy of the law arises from the fact that in every chemical pro- 

 cess the action takes place between molecules, and that the products of 

 the process are new molecules, preserving always, of course, their defi- 

 nite relative weights. The student will thus be brought to the chemi- 

 cal conception of the molecule as the smallest mass of any substance 

 in which the qualities inhere, and he will come to regard a chemical 

 process as always taking place between molecules. 



Thus far nothing has been said about the composition of matter. 

 A chemical process has been defined simply as certain factors yielding 

 certain products, but nothing has been determined about the relations 

 of these several substances except in so far as they are defined by the 

 three laws illustrated above. But now it must be shown that a study 

 of different chemical processes compels us to conclude that in some 

 cases two or more substances unite to form a compound, while in other 

 cases a compound is broken up into simpler parts. Thus, when copper 

 filings are heated in the air, it is evident that the material of the cop- 

 per has united with that portion of the air we call oxygen to form the 

 black product we call oxide of copper ; and again, when oxide of silver 

 is heated, it is evident that the resulting silver and oxygen gas were 

 formerly portions of the material of the oxide. So, when water is de- 

 composed by electricity, the conditions of the experiment show that 

 the resulting oxygen and hydrogen gases must have come from the 

 material of the water, and could have come from nothing else. 



Experiments should now be multiplied until the student has a per- 

 fectly clear idea of the nature of the evidence on which our knowledge 

 of the composition of bodies depends. The decomposition of chlorate 

 of potash by heat, yielding chloride of potassium and oxygen gas ; 

 the decomposition of nitrate of ammonium by heat, yielding nitrous 

 oxide and water ; the decomposition of this resulting nitrous oxide, 

 when the gas is passed over heated metallic copper ; and, lastly, the de- 

 composition already referred to, of water by electricity, are all strik- 

 ing experiments by which the evidence of chemical composition may 

 be enforced. 



The distinction between elementary and compound substances hav- 

 ing been clearly defined by the course of reasoning already given in 

 outline, the next aim should bB to lead the student to comprehend how 

 substances are analyzed and their composition expressed in percents. 

 The reduction of oxide of copper by hydrogen gives readily the data 



